This invention relates to a method for producing an electret filter. More particularly, this invention relates to a highly efficient method for producing an electret filter which is capable of collecting fine particles at a high collection efficiency.
In a known method, an electret filter is produced by forming a film of a starting resin material, subjecting the film formed to a homo charging either before or after stretching the film, splitting the charged film to produce split yarns, and collecting the split yarns to form them into a filter of a desired configuration.
Such a method wherein the film splitting step is carried out subsequent to the homo charging of the film suffer from frequent film tearing since the film strength is decreased by the surface oxidation or minute surface irregularities caused by the homo charging.
The homo charging of the film also results in an increased adhesion of the film to electrodes, which may also lead to film tearing. Once the film is torn, the torn film, which has been electrically charged, easily becomes twined on the roll or the like to induce various troubles in the production process.
It is therefore an object of the present invention to provide a reliable, highly efficient, and quick method which is capable of producing an electret filter having various excellent performances including an excellent collection efficiency with no production troubles.
To achieve the above mentioned object, the present invention provides a method for producing an electret filter comprising the steps of
forming a film comprising a high molecular weight compound,
splitting the film to form split yarns,
subjecting the split yarns to a homo charging, and
producing the electret filter from the charged split yarns.